3D cameras that provide distance measurements to objects and points on objects that they image are well known in the art. Gated 3D cameras comprise a photosensitive surface, such as a CCD or CMOS camera, hereinafter referred to as a “photosurface”, and a gating means for gating the camera open and closed, such as an electro-optical shutter or a gated image intensifier. To image a scene and determine distances from the camera to objects in the scene, the scene is generally illuminated with a train of light pulses radiated from an appropriate light source. Generally, the radiated light pulses are infrared (IR) light pulses. For each radiated light pulse in the train, following an accurately determined delay from the time that the light pulse is radiated, the camera is gated open for a period of time, hereinafter referred to as a “gate”. Light from the light pulse that is reflected from an object in the scene is imaged on the photosurface of the camera if it reaches the camera during the gate. Since the time elapsed between radiating a light pulse and the gate that follows it is known, the time it took imaged light to travel from the light source to the reflecting object in the scene and back to the camera is known. The time elapsed is used to determine the distance to the object.
In some of these 3D cameras, only the timing between light pulses and gates is used to determine distance from the 3D camera to a region in the scene imaged on a pixel of the photosurface of the 3D camera. In others, the amount of light registered by the pixel during the time that the camera is gated open is also used to determine the distance. In 3D cameras in which the amount of light is used to determine distances to the imaged region, the amount of light registered on a pixel is sometimes corrected for reflectivity of the imaged region, dark current and background light. The accuracy of measurements made with these 3D cameras is a function of the rise and fall times and jitter of the light pulses and their flatness, and how fast the gating means can gate the camera open and closed.
Gated 3D cameras that determine distances to objects in a scene that they image responsive to amounts of light registered on pixels of photosurfaces comprised in the 3D cameras are described in PCT Publications WO 97/01111, WO 97/01112, and WO 97/01113, the disclosures of which are incorporated herein by reference.
A gated 3D camera as shown in WO 97/01111 comprises first and second homologous photosurfaces and a light source that illuminates a scene being imaged with the camera with a train of, preferably IR, light pulses. The first photosurface, hereinafter referred to as a “distance photosurface”, is gated on with a short gate following the time that each light pulse in the pulse train is radiated. The portion of light from each light pulse in the pulse train that is reflected by a region of the scene and enters the 3D camera, which is registered on a pixel of the distance photosurface, is a function of the distance of the region from the pixel. The second photosurface, hereinafter referred to as a “normalization photosurface”, is preferably not gated. The portion of light from each light pulse in the pulse train that is reflected by a region of the scene and enters the 3D camera, which is registered on a pixel of the normalization photosurface, is independent of the distance of the region from the pixel. The amount of light registered on the pixel is a measure of the total amount of light reaching the camera from the imaged region. An amount of reflected light registered on a pixel of the distance photosurface from all the light pulses in the pulse train is normalized to an amount of reflected light from all the light pulses registered on a corresponding pixel in the normalization photosurface. Normalized amounts of light are used to determine distances to regions in the scene.
U.S. Pat. No. 5,434,612 to Nettleton, the disclosure of which is incorporated herein by reference, describes a gated 3D camera comprising first, second and third photosurfaces. A scene imaged with this camera is not illuminated with a train of light pulses but with a single light pulse from a laser and the three photosurface are gated with respect to the time that the light pulse is radiated. The first photosurface is a distance photosurface. It is gated with a short gate so that a portion of the light pulse reflected by a region of the scene that is collected by the camera and registered on a pixel of the photosurface is a function of the distance of the region from the pixel. The second photosurface is a normalization photosurface. It is gated with a long gate so that the amount of reflected laser light registered on a pixel of the photosurface from an imaged region is a measure of the total amount of light reaching the camera from the imaged region. The third photosurface is used to measure background light by measuring the amount of light reaching the camera in a band of wavelengths near to wavelengths of light radiated by the laser. A filter that transmits light in the band of wavelengths close to the wavelengths of the laser light but blocks light having a wavelength the same as a wavelength of light radiated by the laser shields the third photosurface. The third photosurface is gated simultaneously with the normalization photosurface by a long gate having a same gate width as the gat that gates the second photosurface. A photosurface used to measure background light is hereinafter referred to as a “background photosurface”.
Amounts of light registered on the background photosurface are used to correct the amounts of light registered on pixels of the distance and normalization photosurfaces for background light. Background corrected amounts of light registered by pixels on the normalization photosurface are used to normalize background corrected amounts of light registered by pixels on the distance photosurface. Distances to regions in the scene are determined from the background corrected normalized amounts of light registered by pixels on the distance photosurface.
Generally photosurfaces used in 3D cameras are gated by an external fast shutter. Certain types of CCD cameras allow for gating image acquisition on and off during a frame by turning the photosurfaces on and off. However, turn-on and turn-off times of these photosurfaces are generally much too long to enable gating the photosurfaces for the purposes of accurate distance measurements by turning them on and off. Typically turn-on and turn-off times for CCD photosurfaces are on the order of microseconds while gating for accurate distance measurements requires turn-on and turn-off times on the order of nano-seconds or less.
An electro-optical shutter suitable for use in 3D cameras, such as those described in the cited patent and patent applications is described in PCT Publications WO 99/40478, the disclosure of which is incorporated herein by reference.
Generally, a 3D camera is used in conjunction with an imaging camera, such as a video camera, that provides an image, hereinafter referred to as a “picture”, of a scene being imaged with the 3D camera responsive to visible light from the scene. The 3D camera provides a “depth map” of the scene while the imaging camera provides a picture of the scene. Distances provided by the depth map are associated with visible features in the picture. In some applications distances associated with a picture of a scene are used to “window” the scene and remove unwanted features and/or objects in the scene, such as for example a background, from the picture. Such applications are described in PCT publication in WO 97/01111 cited above.
PCT patent application PCT/IL98/00476, entitled “Distance Measurement with a Camera”, by some of the same inventors as the inventors of the present invention, the disclosure of which is incorporated herein by reference, describes a photosurface comprising pixels each of which has its own circuit that is controllable to gate the pixel on or off. A single photosurface of this type is useable to simultaneously provide the functions of a distance, background, and normalization photosurface of a 3D camera as well as an imaging camera. However, as the number of functions that the photosurface performs increases, the resolution of the photosurface decreases.
It is advantageous to have a simple robust optical system comprising a 3D camera and an imaging camera that is easily adjustable to simultaneously optimize quantities of light available from a scene imaged by the system that reach the cameras.